1. Field of the Invention
The present invention relates to a mobile communications system, and more specifically to a mobile system between the cells of mobile terminals. A typical example is a handover process during communications in a W-CDMA mobile communications system.
2. Description of the Related Art
The IMT2000 (W-CDMA) system is one of the mobile communications systems, and includes a plurality of base stations (BTS) forming the respective radio areas, and base station control devices (RNC) which are the upper devices to the plurality of base station devices. A mobile terminal can establish communications over a mobile communications network by setting a radio connection with any BTS (base station). When the CDMA system is used, a plurality of communications channels can be set using a diffusion code. However, a diffusion code is finite, and there is a restriction on the number of channels available to a frequency (carrier) due to the problem of an increasing interference frequency. Therefore, a plurality of frequencies (carriers) are assigned to a BTS, and a mobile terminal performs communications using any of the carriers.
When the mobile terminal during communications travels and reaches the boundary of the BTS, it is necessary to perform a handover to an adjacent BTS. A handover can be a hard handover and a soft handover. In a hard handover, a carrier used after the handover is switched to a carrier of a different frequency. In a soft handover, a carrier used after the handover has the common frequency.
In the above-mentioned CDMA mode mobile communications system, various handover technologies have been disclosed. The conventional technologies are listed below.
First, in the first conventional technology disclosed by the patent literature 1, the traffic among a plurality of carriers is assigned to each carrier depending on the load level in the CDMA mode mobile communications system.
In the second conventional technology disclosed by the patent literature 2, only in the CDMA mode mobile communications system, when a base station configures an area using a plurality of carriers, the radius of a cell of only a predetermined carrier is set to be large enough to contact the carrier of the adjacent base station, and the radii of the cells of the other carriers are set to be small enough not to contact the adjacent base stations, thereby suppressing the reduction of transmission power in a small area or the interference of a transmission signal from an adjacent base station.
The third conventional technology described in the patent literature 3 discloses the handover process performed when a terminal travels from the cell A to the cell B when a cell G is located between a cell A and a cell B, the α sectors of the cell A and the cell G overlap in signal intensity at the same frequency, and the β sectors of the cell G and the cell B overlap in signal intensity at the same frequency.
[Patent Literature 1] Japanese Patent Application Laid-open No. 2000-224650
“Improved Assignment of Channels among Plural Carriers in the Spread Spectrum System”
[Patent Literature 2] Japanese Patent Application Laid-open No. 2000-125333
“CDMA Mode Mobile Communications System”
[Patent Literature 3] Japanese Patent Application Laid-open No. 2002-165256
“Handoff Method and System Therein in the CDMA Mobile Communications”
Each of the above-mentioned conventional technologies contributes to the improvement of the reliability of the communication quality of the CDMA Mode mobile communications system by, for example, enhancing the precision of a handover, but the following problems cannot be solved.
That is, when a number of mobile terminal simultaneously start the respective handovers in an area (cell) under a predetermined BTS, the load of the RNC for performing the process of adding and deleting a radio link temporarily increases with the handovers, thereby raising the possibility of the delay of communications and the occurrence of the disconnection of a call. As a result, the communication quality is considerably degraded.
The above-mentioned simultaneous handovers occur when users of mobile terminals of the number exceeding a predetermined number travel together in transportation means such as a streetcar, a train, a vehicle, a bus, etc. A typical example is a case where a number of users traveling with their mobile terminals by streetcar. But, for example, when the users travel by bus, there can be a case in which a plurality of buses happen to be driven at a certain boundary between cells, or a similar case happens when users travel in vehicles and the cells of their mobile terminals includes a part of a freeway. In another case, when users walk, and pass through the boundary of the cells of the terminals in communications, and when a combination of these cases occurs, handovers can simultaneously arise.
FIG. 1 is an explanatory view showing the problem that occurs when users of mobile terminals travel by streetcar. FIG. 2 is a one-dimensional view of the boundary of carriers shown in FIG. 1.
In FIG. 1, a base station (BTS) #1 has an antenna A, a base station (BTS) #2 has an antenna B, and a base station (BTS) #3 has an antenna C. The base stations transmit signals of a plurality of frequencies (in this example, carriers f1 through f4) via the respective antennas. These antennas A, B, and C are arranged along the passage line of a streetcar.
The BTS#1, #2 are controlled by a radio network control station (RNC)#1, and the BTS#3 is controlled by the RNC#2. As shown in FIG. 1, a streetcar carrying the users equal to or exceeding a predetermined number sequentially passes through the communications area under the control of the antennas A, B, and C in the direction of the arrow a.
In FIG. 1, since a plurality of carriers f1 through f4 assigned by the antenna A are transmitted by constant power, the radius of a cell of each carrier transmitted from the antenna is the same. This holds true with the antennas B and C. Therefore, as shown in FIG. 2, the boundaries between the respective adjacent cells are the same for all carriers. In FIGS. 1 and 2, since each BTS uses a plurality of common carriers, for example, the same 4 carriers (f1 through f4), a soft handoff (using the same frequency before and after a handover) can be used for a handover.
In the situation as shown in FIGS. 1 and 2, handovers are simultaneously performed at points (1) and (2) shown in FIG. 2. However, relating to the point (1) in this case, when communicating users exceeding a predetermined number are traveling in a streetcar driven at a speed higher than a predetermined speed for example, at the point (1), all handover processes (adding and deleting a radio link) on all communicating mobile terminals in the streetcar have to be processed by the respective RNC through the RNC#1 during the time of the passage of the mobile terminals from the time of the passage of the user in the leading position in the streetcar at the point (1) to the time of the passage of the user in the trailing position in the streetcar at the point (1). In this case, for example, if an instance ID to a mobile terminal at the start of communications is simultaneously assigned to each mobile terminal, then the highest load is temporarily applied to the RNC.
Generally, a number of BTSs are connected under the RNC, and it is preferable that processes to each RNC are distributed even when only transfer processes to each RNC are considered. Although the overload status to an RNC increases a risk of a handover, the failure of a handover invites a communication delay and a call disconnection, that is, the degradation of communication quality when mobile terminals are traveling at a high speed as shown in the above-mentioned example.